Like Hekri said, vectors are one storage option that can change size at run time, but you can also use new to create a dynamic array. The only problem there is that you would have to determine the size needed before you create the array anyway. Vectors don't suffer this limitation, but if you can ask the user to input how many numbers they will be entering you can use a dynamic array. Vectors require you to learn how to use them, but then that's something you'll have to learn anyway so it's up to you.